Since it is easy to copy digital content (hereinafter referred to simply as content), as well as to alter it, unauthorized users can readily copy or alter such content. Recently, for example, digital music content, such as CD (compact disk) recordings, has become popular as one example of music distribution. In addition, an audio compression technique is also available that can compress data of the music content, without greatly deteriorating the tone quality. Thus, an activity tends to occur that the data of music content, such as on a CD, are compressed and the compressed data are distributed via a network, such as the Internet.
In order to prevent this activity, a method is required whereby it is possible to distinguish between data constituting original content and legal copies thereof from unauthorized copies. For this purpose, electronic watermarking techniques, or electronic signing techniques, have been developed.
Electronic watermarking is a technique whereby predetermined watermark information is embedded, as an additional signal, in content data. Once such watermark information has been embedded in content data, a special computer processing application is employed to detect the watermark information so it can be used for content authentication. That is, when the content data is changed by altering the content, the watermark information detected from the content data is accordingly changed (deteriorated). Thus, the alteration of the content can be identified by examining the state of the watermark information.
Various methods for inserting electronic watermarks into content have been proposed, and various conditions, such as the resistance of the watermark, can be selected by setting the embedding level and the embedding position of the watermark information to the content data. The robustness of watermarks represents the level whereat watermark information, i.e., an additional signal, can be maintained relative to an alteration of content data effected by content processing. That is, an electronic watermark that can be detected even after content data has been greatly altered possesses high resistance to the deterioration that results from processing. Whereas an electronic watermark that can be detected only when little processing or altering of content data has taken place has low resistance to such deterioration.
An electronic watermark that exhibits exceptionally high resistance to the deleterious effects accruing from the alteration of content data and that is resistant to all forms of processing is referred to herein as a robust electronic watermark. While an electronic watermark that exhibits low resistance to the processing of content data and that suffers great deterioration, even when content data is only slightly altered, is herein referred to as a fragile electronic watermark. And an electronic watermark that exhibits an intermediate resistance to the processing of content data, suffering great deterioration when subjected to a specific type of processing but resisting the potentially damaging effects of other types of processing, is herein referred to as a semi-fragile electronic watermark.
When a content creator embeds a robust electronic watermark in content data, the electronic watermark, even after the copying or the processing of the content data, need only be detected for the creator of the original content data to be identified. Further, when a fragile electronic watermark is embedded in content data, since only a little data processing is required for the electronic watermark to be deteriorated, whether the data has been processed can be readily ascertained.
In addition, when a semi-fragile electronic watermark is embedded in content data, it can easily be ascertained whether a specific process, one by which the electronic watermark is easily deteriorated, has been performed. For example, when content data for digital music, such as that which has been recorded on a CD, are compressed, as a result of the compression processing, generally, the amount of data retained in low frequency bands is reduced only a small amount, while in high frequency bands, to reduce the size of the musical content, as great an amount of data is deleted as it is possible without degrading the tone quality. Therefore, when the music content has been compressed, watermark information embedded only in the high frequency bands of the content data is destroyed. Determination as to whether the music content has been compressed can be ascertained by performing an electronic watermark detection process.
When digital music content is distributed via a network, data compression is generally performed in order to reduce the amount of data to be transmitted. Therefore, when a semi-fragile electronic watermark, which suffers deterioration when data is compressed, has been embedded in digital music content, if deterioration of the electronic watermark is detected it is assumed that the music content has been compressed, and the possibility exists that the music content was distributed by an unauthorized user. Furthermore, means for preventing the copying and the alteration of data by unauthorized users has also been proposed that uses both the electronic watermarking technique and an electronic signing technique.
Electronic signing is a technique whereby signature information is added to data to guarantee the legality of content. Assume that a creator, A, of predetermined digital data m employs the electronic signing technique to prove that he or she prepared the digital data m. In this case, first, creator A generates a signature s, comprising digital data that differ from the digital data m, using a signature function S and the digital data m.s=S(m)
Creator A retains the signature function S, a secret function that is not distributed to other parties, and transmits the paired digital data m and signature s to a third party, B. Then, upon the receipt of the digital data m and the signature s, to ascertain whether digital data m was actually prepared by creator A, third party B employs the verification function V and the following equation to determine whether the signature s is correct,determination results (true or false)=V(m,s).
The verification function V returns a logical true when s=(Sm), and a logical false in all other cases. Since this verification function V is distributed by creator A, the function can be obtained by third party B. Finally, when the result obtained by using the verification function V is true, third party B can confirm that the digital data m is correct when the result obtained using the verification function V is true, or that the digital data m is not correct when the result obtained using the verification function V is false.
As is described above, since the signature s that is generated depends on the digital data m, if even one bit of the digital data m is changed, accordingly, the value returned for the signature s will be entirely different. That is, for digital data m′, obtained by altering the digital data m,V(m′,s)is always false. Thus, a change in the data effected by the processing of the digital data m can be detected.
Next, assume that a forger O prepares digital data x and forges a signature s for a creator A, in order to falsely represent that the digital data x was prepared by creator A. Since forger O does not know the signature function S, the forger can not directly calculates=S(x).
And even though theoretically it would be possible for forger O to useV(x,z)to calculate all possible signature values z, and eventually to find a z=s that would return a logical true when used in V(x,z), in actuality, the amount of time that would have to be expended to obtain a value that corresponds to the correct signature s would be enormous, and realistically, such a search would be impossible.
However, while the electronic watermarking or the electronic signing technique can detect the copying or the processing of content, advantageously, more detailed information should be obtained in order to eliminate unauthorized copying and distribution. For example, since the above described music content may be distributed or copied legally via a network, means is required that can distinguish between authorized copying and distribution and unauthorized copying and distribution.
When the fragile electronic watermark that is easily destroyed by copying or alteration is used, the possibility that content data has been copied or processed can be detected. However, in this case, the content in which the electronic watermark is embedded and that is being distributed can not be distinguished from the content in which the electronic watermark was originally embedded.
Therefore, to prevent the unauthorized copying and distribution of content, simple detection performed to determine whether the content data have been changed is not sufficient, and a technique is required that satisfies the following conditions:                1. The content is a target the alteration of which can be detected.        2. The content has been changed and the type of change can be identified.        3. The copying and alteration of the content can not be hidden.        
The hiding of copying and changing of content is a process used to disguise content that has been copied or altered and make it appear to be content that has not been copied or changed.
In the above description given for music content, the watermark information embedded in high frequency bands is destroyed during the compression of the content data. However, when the same data as the watermark information embedded in the high frequency band is obtained by using a specific method, and is added to content data that has been decompressed, the watermark information is distinguished from compressed content data, so that in appearance this data does not differ from the content data that has not been compressed. Therefore, this processing is effectively avoided to prevent the unauthorized copying and distribution of content.
Conventionally, the type of processing used for content is identified, to a degree, by embedding in the content multiple types of electronic watermarks having different resistances. However, as is described above, processing performed to hide the copying and handling of the content can not be prevented. The copying or the processing of content is not easily performed by using electronic watermarking and another encryption technique; however, basically, the processing performed to hide the copying or the handling of content can not be prevented.
When unauthorized copying or processing is to be prevented by using electronic signing, even a slight change in the content data is detected, while on the other hand, even authorized processing can not be performed for the content data. Further, since the determination results are false for all changes of content data, the processing type and the processing level can not be identified.